How Long Would A Fidget Spinner Spin In Space?

Like most parents of young children, I've been forcibly made aware of the "fidget spinner" fad. Despite FiveThirtyEight declaring them "over," my kids brought home two new ones from a birthday party on Saturday, adding to three others that we currently own, plus a couple more that were lost at school.

Some of the expanding collection of fidget spinners at Chateau Steelypips. Photo by Chad Orzel.Chad Orzel

So, the question asked in the Mashable piece is whether a fidget spinner would spin forever in space. They end up saying "no," but as with most questions involving physics, this ends up depending on your definition of "forever." And also exactly how the test is being done -- as I noted on Twitter, the big question is whether someone is holding the spinner, or whether it's been let go to float freely in space. If the astronaut testing it is holding the spinner in their hand, I would expect it to stop in more or less the same amount of time required on Earth, but if it's floating freely, it should spin for a very long time indeed.

Explaining why that is requires a bit of thought about what makes the fidget spinner stop in the first place. The key issue is angular momentum, the physical quantity associated with rotational motion. Noether's Theorem tells us this is a consequence of rotational symmetry in the laws of physics, which means that the total angular momentum of the universe should not change over time. Thus, when you start something spinning, that angular momentum can't just vanish spontaneously, it has to be transferred to something else.

For a fidget spinner on Earth, this transfer happens because the spinner generally has to be held up against gravity, which means it's in contact with something much larger than itself -- the hand of a person holding it, or a surface that it's sitting on. This has the effect of holding the central ring of the spinner immobile, with the outer bits spinning relative to it on a very good roller bearing. As good as that bearing is, though, there's some friction between the spinner and the central ring. This slowly brings the spin to a stop -- in angular momentum terms, the friction of the bearing causes a torque on the spinning bit that transfers the angular momentum of the initial spin to whatever is holding it. On Earth, that means the person holding it, or the table it's on, which in turn is braced against the ground, so in the end, the rotation of the entire Earth increases or decreases by an infinitesimal amount as the spinner comes to a stop.

In this frame from NASA TV, a SpaceX Dragon approaches the International Space Station on Monday, June 5, 2017, making an unprecedented second trip to the orbiting outpost. The Dragon supply ship, recycled following a 2014 flight, was launched from Florida on Saturday. (NASA TV via AP)

If the spinner is held by an astronaut floating in space, I would expect roughly the same behavior: over about the same amount of time it takes for an earthbound fidget spinner to come to a stop, the angular momentum of the original spin would be transferred to the astronaut. In principle, this should cause a free-floating astronaut to start rotating, but since the mass of an astronaut is around a thousand times that of a fidget spinner, and their length between 10-100 times as big, they'd be spinning at something like a millionth the rate of the initial spin, which you'd never notice.

The more interesting case is where an astronaut starts the fidget spinner spinning, and then lets it go, so it's freely floating. In that case, you still have friction in the bearing, but now that central ring isn't anchored. So all the friction can do is equalize the rate of spin of the outer spinner and the inner ring of the bearing. This will lead to a small reduction in speed for the spinner, and a large increase in speed for the central ring, until the whole thing is spinning as a unit.

Once you get to that point, you need some other force to transfer the angular momentum of the spinning fidget spinner to...something else. If you're inside some kind of space habitat like the International Space Station, the obvious source for such a force would be drag from air resistance, which would slowly transfer angular momentum from the spinner to the air molecules inside the ISS (and then eventually to the ISS itself). I'm not sure what the time scale for this would be, but it'd certainly be much longer than what you see from an earthbound spinner. Air resistance plays some role in stopping a spinner on Earth, but it's probably a small contribution relative to the frictional torque. The test spinner I grabbed from my kids' pile of them ran for a solid three minutes before coming to a stop, so if I had to guess, I'd say something on the order of a few hours for air resistance alone to stop a spinner floating in the ISS.

Put it outside the ISS, though, in the much more diffuse atmosphere there, and it'd go for a whole lot longer. This order-of-magnitude guide to pressure says low Earth orbit is about one ten-trillionth of atmospheric pressure, so I would roughly expect the spinner to require ten trillion times the original stopping time if that drag force were the only thing acting to slow it down. That'd be something in excess of 57 million years (ten trillion times the 3-minute stopping time I see on Earth), probably more like a few billion years. It would eventually stop, but there are decent odds that the expansion of the Sun would destroy the spinner (and, incidentally, the Earth) long before that.

If you take air resistance out of the equation, the next possible candidate would be tidal gravity -- the fact that the force of gravity is very slightly larger on the edge of the spinner closer to the Earth than the farther side. This difference, on a much larger scale, is what causes the tides on Earth, and is responsible for the "tidal locking" that keeps one face of the Moon pointed toward Earth at all times. It also transfers some angular momentum from the Earth to the Moon -- the Earth's rotation is slowing very gradually (which is why we occasionally add a "leap second" at New Year's), with the "lost" angular momentum going into increasing the distance between the Earth and the Moon very slightly. Both the rotation and the Earth-Moon distance are tracked very precisely, so these are very real and well-understood effects.

How long would tidal gravitational forces take to stop a fidget spinner? I have absolutely no idea, but I'd guess it's even longer than air drag at ISS altitudes.

So, while in a strict technical sense a fidget spinner set spinning and released in space would eventually come to a stop, that's probably not very meaningful on a human scale. If it's not being held by an astronaut or something with enough mass to keep the central ring stationary, I'd expect its spin to long outlast the whole of human civilization, let along a passing fad like fidget spinning.

I'm an Associate Professor in the Department of Physics and Astronomy at Union College, and I write books about science for non-scientists. I have a BA in physics from Williams College and a Ph.D. in Chemical Physics from the University of Maryland, College Park (studying la...